Microspectrometer gas analyzer

ABSTRACT

A robust, compact spectrometer apparatus for determining respective concentrations or partial pressures of multiple gases in a gas sample with single as well as multiple and even overlapping, absorption or emission spectra that span a wide spectral range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 as aContinuation-In-Part (CIP) from U.S. patent application Ser. No.10/939,279, filed Sep. 10, 2004, now U.S. Pat. No. 7,157,711, whichclaims priority under 35 U.S.C. § 120 as a CIP from U.S. patentapplication Ser. No 10/227,135, filed Aug. 23, 2002, now U.S. Pat. No.6,791,086, which claims priority from U.S. Provisional Application Ser.No. 60/316,763, filed Aug. 31, 2001 under the provisions of 35 U.S.C. §119(e), the contents of each of which are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for efficiently androbustly measuring gas concentrations/partial pressure of respiratoryand anesthetic gases.

2. Description of the Related Art

It is well known by those of ordinary skill in the art that gasanalyzers of the nondispersive infrared (NDIR) type operate on theprinciple that the concentration of specific gases can be determined by(a) directing infrared radiation (IR) through a sample of a gaseousmixture, (b) separately filtering this infrared radiation to minimizethe energy outside the band absorbed by each specific gas (c) measuringthe filtered radiation impinging upon one or more detecting devices and(d) relating a measure of the infrared absorption of each gas to itsconcentration. Gases that may be measured exhibit increased absorption(and reduced transmittance) at specific wavelengths in the infraredspectrum such that, the greater the gas concentration, theproportionally greater absorption and lower transmittance. An extensionof this NDIR technique uses a continuous, linear bandpass filter,followed by a linear array of detectors.

Gas analyzers are widely used in medical applications and may becharacterized as being located either in the main path of the patient'srespiratory gases (mainstream analyzers) or in an ancillary path usuallyparalleling the main path (sidestream analyzers). A mainstream analyzeris situated such that the subject's inspired and expired respiratorygases pass through an airway adapter onto which the analyzer is placed.Mainstream designs require the optical and electronic components to beinterfaced to a patient's airway or to a respiratory circuit incommunication with a patient in a location in relatively close proximityto the patient. As a result, to be accepted in clinical use, themainstream gas analyzer must be designed as a compact, lightweight yetrobust structure unaffected by typical mechanical abuse and temperaturevariations associated with prolonged use in health care facilities.

While conventional mainstream gas analyzers work well for a small numberof specific, non-overlapping spectrum wavelengths, it is difficult tochange wavelengths of interest. The system becomes increasinglyinefficient if there are more than 2 or 3 wavelengths of interest, andit is very difficult and expensive to provide resolutions significantlybetter than 0.1 micron, FWHM (full-width at half maximum) in the IRregion.

It is known to use grating spectrometers for gas analysis. There are twogeneral configurations of grating spectrometers: the spectrograph, whichoriginally spreads the spectrum out over a strip of photographic film ora linear array detector, and the spectrometer, which uses a singledetector that is set at an appropriate location or angle to register aparticular spectral element.

For IR gas measurements, an IR source provides broadband energy that iscollimated and passed through a gas sample cell. The collimatedbroadband energy, now attenuated at certain wavelengths, is directed toa diffraction grating where it is diffracted from the grating, spreadout into a continuous spectrum, and focused with a mirror onto a smalldetector. The diffraction grating is rotated about an axis parallel tothe grating lines, and substantially coaxial with the face of thediffraction grating. As the diffraction grating is rotated, the spectrumis scanned past the single detector. Since the diffraction gratingrotation is synchronized with the detector readout electronics,specific, but arbitrary, spectrum features can be isolated andregistered.

It is axiomatic that a microspectrometer should be small andlightweight. The present invention contemplates, for example, that themicrospectrometer is made small and lightweight enough to be useddirectly on a patient airway, i.e., mounted in a mainstream fashion on apatient circuit. While the optics can, in general, be made small enoughto suit the purpose, it is difficult to make the mechanism that drivesthe diffraction grating, that is, the spectrum scanner, sufficientlysmall to suit this purpose. Currently available electromechanicalscanner drives that are much too large, mostly too heavy, require toomuch power, and cost too much to be used in this manner.

For example, many conventional spectrometers rotate the diffractiongrating using a motor of some sort, oscillating linkages to drive thediffraction grating from the motor, and a bearing assembly. While suchan arrangement can deliver good results, such a structure is relativelylarge, heavy and expensive. Other conventional spectrometers use anoscillating motor, sometimes called a galvanometer drive, in place ofthe motor and linkage. Such arrangements are less expensive, but stilllarge, heavy and relatively expensive.

U.S. Pat. Nos. 6,249,346 (2001) to Chen, et al., 6,039,697 (2000) toWilke, et al., and 5,931,161 (1999) to Keilbach, et al. all discloserelatively smaller sized spectrometers, but of designs that are of unduebulk and, in some instances, complexity.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aspectrometer that overcomes the shortcomings of conventional gasanalyzing devices. This object is achieved according to one embodimentof the present invention by providing a robust spectrometer apparatusfor determining respective concentrations or partial pressures ofmultiple gases in a gas sample with single, as well as multiple and evenoverlapping, absorption, or emission spectra that span a wide spectralrange.

The present invention adapts a grating spectrometer for use in a compactrespiratory gas analysis instrument. Specifically, the present inventionemploys a scanning spectrometer, which scans, or sweeps, the spectrumacross a fixed detector. From an optical point of view, this apparatusmay be characterized as a modified Ebert scanning monochrometer.

A very small, inexpensive oscillating mirror may be made using a MEMS(MicroElectroMechanical System) fabrication process. With a diffractiongrating added to the mirror surface, this structure provides a very lowcost, small, lightweight but rugged scanner for an in-line IR gasanalysis instrument.

Spectrum resolution is primarily a function of the grating size,aperture, line pitch, diffraction order, and collimation. In the presentinvention, the required grating width is in the 1 to 2 mm range, whichis well suited to existing MEMS technology. The other parameters areeasily obtained or controlled, at least well enough for necessaryaccuracy.

The diffraction grating may be formed separately and glued on to the“mirror” surface or, preferentially, the diffraction grating may beformed in the surface of the mirror as part of the MEMS fabricationprocessing. A hologram type of grating may also be used. The drive tomake the mirror oscillate may be magnetic, wherein the mirror either hasa planar coil formed on the back or the mirror itself is made magneticor, alternatively, the mirror may be driven electrostatically. Becausethe required angular amplitude is relatively small, an electrostaticdrive is currently preferred.

The apparatus of the present invention may also be configured in severaladditional ways. In one instance, the oscillating grating may be removedand replaced by a scanning (oscillating) mirror. In an embodiment ofthis approach, the mirror scans the input light over a fixed grating,which disperses the spectrum. As before, the spectrum is focused by amirror onto the detector plane. While this alternative method requiresone additional component, the manufacturing cost may be less because theMEMS oscillating element does not need to have a grating fabricated onits surface.

In yet another alternative embodiment, the oscillating mirror may bepositioned to direct the attenuated broadband energy beam back throughthe gas sample cell, with the grating and detector on the same side ofthe gas sample cell as the IR source. The advantage of this arrangementis higher sensitivity (due to the double pass through the gas in thecell), and a somewhat narrower package. Alternatively, in the doublepass configuration, the mirror on the side opposite to the source may befixed, and an oscillating mirror/fixed grating (or oscillating grating)and detector system located on the source side. These variousembodiments may be configured in a single plane or the oscillatingmirror, scanning grating or a focusing mirror may be rotated inorientation to direct the beam in a different plane, so that differentpackage configurations may be easily accommodated.

A diffraction grating can provide diffracted beams in several orders.Ordinarily, the first order is used, either + or −1, and the shape ofthe grooves in the grating are designed to emphasize the chosen order.However, there can be some residual energy in higher orders. The resultis that spectral regions at a shorter wavelength may overlap the firstorder spectrum. This problem may be solved, as required, with a blockingfilter set to cut off all wavelengths that are outside of a spectralregion of interest.

Data processing electronics for the apparatus of the present inventionare synchronized with the motion of the scanning element. One approachis to extract a timing signal from the mirror drive. Alternatively, themirror may have coils or magnetic or piezoelectric sensors mounted on itto provide signals indicative of a substantially instantaneous locationof a portion of the mirror for use in synchronization. Another sensingtechnique for using in synchronization is to reflect an auxiliary beamoff the front or back of the mirror to a separate detector. A currentlypreferred technique is to use a unique feature of the detected spectrum,if such is available or provided. Assuming that the mirror is resonant,there will be relatively long periods when the detector will not receiveany signal. This is because the scan will be more easily interpreted ifit is in the more nearly linear part of the scan, and because theblocking filter will remove all signals prior to, or following, thespectral region of interest. As such, the long blank period followed bya sharp rise in signal may be used to provide a suitably unique markerto a phase lock loop synchronizer. The blank period also provides abackground light condition so that the detector zero may be set. Fullscale can be implied by any spectral region between absorption peaks, orregions where known peaks have been subtracted.

Note that because the data generated by the apparatus is continuous, itis believed to be possible to incrementally subtract known, andpreviously stored, specific spectral lines, i.e., “peel off” individuallines, one by one. Such processing improves separation, or reducesinterference, especially of weak lines.

These and other objects, features and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic optical system layout for a spectrometer with anoscillating scanner mirror-diffraction grating combination according tothe principles of the present invention, and FIG. 1B is a schematicdiagram of the spectrometer in which the optical system of FIG. 1A canbe suitably employed;

FIG. 2 is a perspective view of an oscillating mirror/gratingcombination suitable for use in the optical system of FIG. 1A;

FIG. 3 is a schematic optical system layout for a spectrometer with afocusing mirror-diffraction grating combination according to the presentinvention;

FIGS. 4A through 4F are schematic illustrations of a number of exemplarylayouts for spectrometers using collimated light beams, enablinganalysis of a plurality of spectral bands in accordance with theprinciples of the present invention;

FIGS. 5A through 5C are schematic illustrations of a number of exemplarylayouts for spectrometers using non-collimated light beams, enablinganalysis of a plurality of spectral bands in accordance with theprinciples of the present invention;

FIGS. 6A-6D are schematic illustrations of further exemplaryarrangements for the spectrometers in accordance with the principles ofthe present invention

FIGS. 7A and 7B are top and bottom perspective views of anelectro-mechanical scanner drive according to the principles of thepresent invention;

FIGS. 8 is a schematic diagram of a circuit for performing an automaticscanning frequency adjustment according to the principles of the presentinvention;

FIGS. 9A and 9B are waveforms showing the return signals for the scannerdrive during resonance and an non-resonance, respectively.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

FIG. 1A is a schematic optical layout for a spectrometer according tothe principles of the present invention. Energy in the form of a lightbeam 10, such as an infrared beam, proceeds from a sample cell G (seeFIG. 1B) and strikes a turning mirror 12. Turning mirror 12 thenreflects light beam 10 towards scanning grating reflector 14, which mayalso be termed a scanning mirror. It should be noted that scanninggrating reflector 14 oscillates about an axis perpendicular to the page(the oscillations are shown in an exaggerated form). From the scanninggrating reflector 14, the now-dispersed light beam 10 travels to afocusing mirror 16 which, in turn, focuses light beam 10 to the detector18 which includes, or has associated therewith, appropriate readoutcircuitry. Detector 18 may comprise, for example, a slit- orpinhole-defined detector, as known in the art.

FIG. 1B schematically illustrates the complete the structure of aspectrometer for use with the various optical embodiments of the presentinvention. As shown in FIG. 1B, an infrared light source S emits aninfrared beam which may be collimated using source optics or acollimator C, as shown. The collimated infrared beam then enters gassample cell G, exiting same to turning mirror 12. Such an arrangementmay be used with all of the described embodiments herein, except it isnotable that the embodiments of FIGS. 5A through 5C do not require thepresence of a collimator C or source optics to collimate the infraredbeam.

Referring to FIG. 2, scanning grating reflector 14 has diffractiongrating lines 22 positioned on it. The lines may be glued on or machinedinto the reflective, mirror surface using a MEMS process, or they may bepositioned through some other known technique. U.S. Pat. 6,201,269 toMcClelland, et al., the disclosure of which is incorporated herein byreference, discloses a suitable MEMS process for fabricating anoscillating mirror, which process may be adapted to fabricate scanninggrating reflector 14. The grating can also be made in the form of ahologram.

Scanning grating reflector 14 has a flexure axis 24 parallel todiffraction lines 22 and is mounted to a frame 26 through supportmembers coaxial with flexure axis 24. Backings 28 may be electricallyconductive so as to provide an electrostatic drive for scanning gratingreflector 14 when leads 20 are connected between backing 28 and asuitable power source P as known in the art. Two power sources P aredepicted for simplicity in FIG. 2 although, of course, a single powersource P may be used to power backings 28 in alternation.

The schematic illustrated in FIG. 1A uses scanning grating reflector 14as both scanner and diffraction grating. However, it is not necessary toinclude the diffraction grating on the scanner. The diffraction gratingmay be scanned in angle by a mirror scanner instead. As shown in FIG. 3,a mirror scanner 32 is used to sweep the input beam 30 from the gassample cell over the diffraction grating and mirror combination 34. Themirror employed in diffraction grating and mirror combination 34 is afocusing element that directs and focuses dispersed energy from mirrorscanner 32 to the detector 36. The image formed is of the defining inputaperture, in the wavelength selected by the diffraction grating andmirror combination 34. In a conventional Ebert monochrometer, there is aslit at the entrance to the monochrometer that defines the aperture tobe imaged. In the present invention, the defining aperture may be thesource, or it may be a separate aperture near the entrance to thescanner/detector assembly. It should be noted that the turning mirror 12of the embodiment of FIG. 1A does not have a structural counterpart inFIG. 3, as the turning mirror is not a required component of theinvention, but is common in the prior art and use thereof does provide anumber of other configuration possibilities.

As another alternative configuration, the mirror-grating function may besplit up, such that the scan is directed to a flat grating mirror,followed by a focusing element, usually a mirror in this IR wavelengthregion, followed by the detector. The advantage of such alternativesplit configuration over the FIG. 1A configuration is that the scanningmirror device is directly manufacturable by presently known processes,while forming a grating on the mirror is not conventional. In contrast,forming a grating on a focusing element by molding techniques isconventional. The disadvantages of the split configuration are that thegrating must be somewhat larger (because the beam moves across thegrating in order to change the angle), and the mirror may need to be anasphere. These are minor issues if, as expected, the grating-mirror ismade by a molding or casting process.

The embodiments described with respect to FIGS. 1A and 3 provide aneffective way to collect spectral data over a wavelength octave.However, these embodiments are designed with a single band, such as, forexample, the 3 to 5 micron band, in mind.

The range of a grating spectrometer is limited in a practical sense toan octave, because of multiple orders. That is, a particular wavelengthwill diffract at a certain set of angles, which depend on thewavelength, the grating period, and an integral number known as theOrder. Because the dispersion is a function of the Order, multipleorders can overlap at the detector plane, making spectra difficult tointerpret. In practical grating spectrometers, the grating is made sothat most of the diffracted energy is directed to a particular desiredorder. This is done by contouring the surface at each groove of thediffraction grating so that light striking that point will be reflectedin the same direction as the desired diffraction order. This contouringprocess is referred to as blazing. In addition, blocking filters may beadded at the spectrometer input or at the detector that will blockwavelength regions that might otherwise cause confusion.

In addition to the 3 to 5 micron band described earlier, it isadvantageous for the present invention to measure the 7 to 10 micronrange simultaneously. The problems in this longer wavelength range arethat, first, a more expensive detector is required, second, transmissionoptics, e.g., lenses, for beam manipulation tend to be more expensive(although a longwave pass filter or function is unavoidable) and third,the second order of the 3 to 5 micron band would tend to fall in thesame plane as the 7 to 10 micron band.

Seven exemplary approaches to optical arrangements for the measurementof additional bands are shown in FIGS. 4A-4F. Note that in allillustrated embodiments shown in FIG. 4A-4F, the input beam has alreadybeen collimated, either by the source optics, or by other conventionalmeans. Note also that the drawings are schematic, i.e., the diffractionangles are illustrative and not exact.

In the embodiment of FIG. 4A, a scanning mirror 42 directs the inputbeam 40 to a dichroic beam splitter 44, which divides the beam into twobands, e.g., 3 to 5 and 7 to 10 microns, respectively. Two separatescanning diffraction gratings 46 disperse the bands; each grating 46being optimized for a respective band. After dispersion, each band ofthe beam is directed by a focusing mirror 48 onto an aperture of adetector D.

In the embodiment of FIG. 4B, a scanning diffraction grating 46 isemployed, and the resultant dispersed beam is divided by a dichroic beamsplitter 44 into two bands. In this case, the scanning diffractiongrating 46 has been optimized for the 7-10 micron band in first order,and also for the 3-5 micron band in second order.

FIG. 4C illustrates an embodiment including a scanning mirror 42,followed by a dichroic diffraction grating 47 that is coated to reflectone band, such as 7-10 microns, and transmit the other. As in othercases, the dichroic diffraction grating 47 would be arranged for firstorder 7-10 microns, and second order 3-5 microns. Alternatively, areflective diffraction grating (non-transmissive) may be employed, and aband splitter located after the diffraction grating.

The embodiment of FIG. 4D uses back-to-back scanning diffractiongratings 46 that only reflect, and together are used as the scanningelement. Band splitting is effected by a dichroic beam splitter 44before the gratings. In this embodiment the gratings may be individuallyoptimized for best performance in specific bands.

The embodiment of FIG. 4E is arranged to provide detection in threebands. The scanning mirror 42 illuminates two reflection/transmissiondichroic diffraction gratings 47 in series. While this arrangementcauses some restrictions on wavelength band placement, it is physicallymore compact than that of FIG. 4F.

The embodiment of FIG. 4F includes a three-dimensional arrangement ofmirrors and gratings that can provide six bands (as shown), and morebands by extension. The input beam 50 is first split into threewavelength blocks of two contiguous octave bands each using multipledichroic or bandpass filters 51, which wavelength blocks are thenscanned by a scanning mirror 52. The axis of the scanning mirror 52 isin the plane of the drawing sheet. The wavelength blocks aregeometrically separated by angle in a plane that includes the mirrorrotation axis. After scanning, the wavelength blocks go to threediffraction gratings 56, each similar to that FIG. 4C but suitablytilted to match the separation angle. Note that only one grating 56, andno detectors, are shown for simplicity and clarity of illustration inFIG. 4F, although such would be included in practice.

FIGS. 5A-5C depict additional embodiments of the present inventionwherein, unlike those described above, light entering the spectrometermay be diverging or converging and the optics modified to compensatetherefore.

FIG. 5A schematically depicts a system wherein light from a source Spasses through gas sample cell G and is reflected, dispersed by agrating and scanned on a scanning flat grating mirror 60. The resultantdispersed light beam is focused using a concave mirror 62 onto detectorD.

FIG. 5B schematically depicts a system using a flat scanning mirror 64,and the scanned beam is reflected to a concave grating mirror 66 thatdiffracts and focuses the light beam onto detector D.

FIG. 5C schematically depicts a system wherein the scanning, dispersionand focusing functions are consolidated into a single element 68 in theform of a scanning mirror which includes a diffraction grating and isconcave for focusing the light beam onto detector D.

As will be understood and appreciated by those of ordinary skill in theart, adding functions to a scanning element increases the cost thereof,but in each instance other elements in a system may be reduced in cost,or eliminated entirely. In particular, the embodiments of FIGS. 5A-5Celiminate the need for collimating elements, and the embodiment of FIG.5C eliminates the need for a separate focusing mirror. Such reductionsin the number of required components enable the fabrication of a lesscostly system due both to elimination of components and a reduction inassembly time.

It will also be understood and appreciated by those of ordinary skill inthe art, the approaches illustrated in FIGS. 5A-5C may be applied to theembodiments of FIGS. 4A-4F for the measurement of multiple bands ofinterest. For example, the components and arrangement of FIG. 5A may beadvantageously employed to modify the systems of FIGS. 4B and 4D, whilethe components and arrangement of FIG. SB may be advantageously employedto modify the system of FIG. 4A, in each instance resulting in theelimination of a focusing mirror. The components and arrangement of FIG.5B may also be employed in the systems of FIGS. 4C, 4E and 4F, althoughthe focusing mirror and grating element would be more complex, since itwould be required to focus in both reflection and transmission. Thefirst, or reflecting, face would be concave, while the second face wouldcomprise a convex refracting face.

In the embodiments described above, two different bands, i.e., the3.5-4.5 micron and 7-9 micron bands, are separately dispersed using thefirst and second Orders of the grating. Filters on the two detectorsmake sure that the respective detectors only react to the proper band.The present invention also contemplates using different Orders of thegrating provide for (essentially) non-contiguous bands that cover a muchlarger range of wavelengths than could be obtained by a single ordergrating.

The invention described above also discloses the use of a dichroicsplitter to direct different bands or segments of a band to twodifferent detectors to the detector(s). The present invention alsocontemplates using a non-wavelength sensitive splitter, i.e., anordinary partially reflective splitter. In which case, appropriatefilters can be provided on or before the detectors to sort out theproper bands.

The above-described embodiments of the present invention use a focusingmirror to form an image at the detector. This focusing function couldalso be performed with a lens formed from any suitable material. Thepresent invention further contemplates that the dichroic splitter can bea partially reflective splitter. In addition, the splitter (reflectiveor transmissive) can be disposed after the focusing mirror (or lens),and before the two detectors.

One function of the microspectrometer of the present invention is toperform a spectral scan of anesthetic agents in the 8 to 10 micron IRband, and concurrently, a scan of the mid-IR CO₂ and N₂O band. Theselection of a basic structure for a spectrometer is easy, due to theexistence of numerous historical systems, e.g., Ebert, Czemy-Temer,Fastie-Ebert, etc, and single or multiple holographic grating systems.The primary system problem is efficiency, that is, how much of thesource light can be deposited on the detector vs. the spectralresolution of the system.

In all systems, the source, or an aperture illuminated by the source, isimaged onto the sensor plane. The size of this image, set by aberrationsand optical magnification, must be less than the desired spectralresolution of the system. Because the resolution is set by the grating,the effective source size is critical. In a typical spectrometer system,the entrance slit is at the focus of a large aperture mirror. The mirrorcollimates the light onto a grating. Diffracted light from the gratingis refocused onto the sensor by a second concave mirror. Because theaperture is large, i.e., a small f number, the efficiency can be large.In the microspectrometer of the present invention, light from the sourcemust first pass through the airway adapter (sample cell) that, in theabsence of added optics, would prevent a large aperture/high efficiencysystem. Even if the beam from the source were to be collimated throughthe adapter, the source size would make the beam spread too large forpractical optics in the spectrometer.

As shown in FIGS. 6A-6D, the present invention solves this problemthrough the use of a large aperture lens at the source that forms anaerial image in the middle of the adapter, i.e., sample cell. A lens 103at the entrance to the detector block will roughly collimate that lightdirectly on to a grating. Lens 103 has a focal length that is aboutequal to the image distance of the lens proximate to the source. Lens103 collimates the beam, and because it is working from an image of thesource, lens 103 tends to collimate the angle of the off-axis beams. Theaction is similar to that of a field lens. Therefore, the beam spread atthe grating is less, and very much less at the following elements.

In FIGS. 6A and 6B, diffracted light from grating 106 is focused by anaspheric mirror 108 on to a sensor (detector) 110. By this technique,the source magnification is kept usably small, and the efficiency ishigh. The present invention contemplates that the lenses are coated withsilicon, as it is the cheapest lens material with a reasonably goodenvironmental stability for this wavelength range. In FIGS. 6C and 6D, afolding or turning mirror 109 is used in place of a concave focusingmirror 108.

FIGS. 6A-6D illustrate three alternative lens configurations. FIG. 6Aillustrates an embodiment that uses a spherical lens 100 provided on oneside of an adapter 102, which is also referred to as the sample cell.FIG. 6B illustrates the use of an aspheric lens 104 with adapter 102.FIG. 6C illustrates a focusing lens 107 provided before folding mirror109. FIG. 6D illustrates a focusing lens 111 provided after foldingmirror 109. The present invention also contemplates providing focusinglens before and after the folding mirror. The remaining components ofthe system, such as source, relective grating 106, and detectors 110,can be configured in any of the arrangements contemplated by the presentinvention, including the specific examples discussed above.

The wavelengths of interest are about 8 to 9.5 microns for the agents,and 4 to 4.7 microns for CO₂ and N₂O, and with reference channels at 3.7and 7.4 microns. The present invention contemplates that, the sameoptics and grating can scan both regions simultaneously, where the IRuses the grating first order, and the mid-IR uses the grating secondorder. A dichroic splitter is needed to separate the detectors.

The scanning rate for grating 106 is preferably in the 100 Hz to 300 Hzrange. One hundred Hz is an approximate lower limit that is set by therequired CO₂ bandwidth, i.e., 10 Hz. The upper limit is set by the IRdetector response time, and mechanical constraints on the gratingactuator. The spectrometer grating range of motion is about +/−5 degrees(mechanical) to cover the range including the reference channels, plusabout 15% to 20% for turn-around. If the reference function is done someother way, or the grating spacing reduced, the range of motion may becut to +/−3 degrees. In an exemplary embodiment of the presentinvention, the grating mirror is about 6 mm wide and 10 mm tall. Thesespecs are well within an inexpensive state of the art at sinusoidalfrequencies of 200 Hz-300 Hz range. A PbSe detector is used for themid-IR because it is fast, sensitive, cheap, and familiar. The IRdetector candidates are MercuryCadmiumTeluride (MCT), microthermopile,microbolometer, or pyroelectric.

The spectral data that will be collected by the MicroSpectrometer shouldinclude reference data on the noise floor (zero signal), sourceintensity (signal span, i.e., clear channel), and spectrum spancalibration. Calibration can be done by reference to the CO₂ line and toan edge filter. Calibration in either band, or calibration betweenbands, is valid for both since the same scanner serves both. Signal zeroand span need to be done on each separate sensor, so a clear channel andblocking function are required on each.

The present inventor recognized that in operation, the scanner thatrotates the diffraction grating runs at a single frequency, and have afixed scan angle. These requirements suggested to the present inventorthat a resonant scanner would be an appropriate system for driving thediffraction grating. A resonant type of scanner drive system has severaladvantages: 1) the power requirements are minimized, assuming a highmechanical Q; 2) the motion of the scan tends to be an exact sinusoidwith minimum harmonics; and 3) an accurate synchronizing signal can bederived from the drive circuit. A resonant scanner drive system doeshave a disadvantage in that the resonant frequency is dependent on theinertia (mass) of the whole moving system, and the magnitude of therestoring force (spring). If either change with time, temperature, ormanufacturing variables, the resonant frequency will change.

The hurdles faced when attempting to use a resonant scanner drive systemis to design a system where the inertial parts, other than the grating(which is fixed by the optical requirements), are minimized, airentrainment is minimized (to keep the mechanical Q high), and theoverall size is minimized. Further, the system, as a whole must acceptsome variation in the resonance.

The present invention addresses these issued and provides a scannerdrive system 200 as shown, for example, in FIGS. 7A-9. Scanner drivesystem 200 includes a taut band 202 that provides the rotational axisfor a diffraction grating 204. It should be noted that the diffractiongrating is omitted from FIG. 7A so that the features of the scannerdrive system under the grating can be viewed. Band 202 also provides aspring return and a mechanical support for the moving components of thescanning system. Grating 204 is fastened on one side and generally atthe center of band 202. A permanent magnet 206 is fastened on the otherside of the band. Spacers 208 are provided on each side of band 202, sothat as the assembly oscillates, the twisting band will not contacteither grating 204 or magnet 206.

Band 202 is supported at its ends by a frame 210, which in an exemplaryembodiment is square. During manufacture, the ends of band 202 arefirmly attached to the frame while under tension. The present inventionalso contemplates holding frame 210 under compression during theprocess, so that the net tension in the band after attachment to theframe is predictable. The present invention contemplates attaching band202 to frame 210 using a spot-weld, solder/braze, or glue if theattachment is re-enforced by bending the band over the outer edge of theframe.

In an exemplary embodiment, band 202 is 0.001″ thick, 0.9 mm wide, andhas a free length of about 7 mm. Grating substrate 2040 is glass, 2 mmthick, and 6 mm in diameter. The resonant frequency is about 200 Hz,depending on the tension in the band, and the proximity of the drivepole-pieces to permanent magnet 206.

Permanent magnet 206 is advantageously a Neodymium-type, which providesan especially strong magnetic field for the size and mass. The magnet ismounted, as stated, on band 2020 (with spacers 208) with the magneticpole axis normal to the plane of the grating, i.e., the magnet surfacethat is attached to taut band is a pole. It may be either North orSouth, but a convention should be set during manufacture, because thephase of the oscillation relative to the driving pulse (see below) willbe dependent on the polarity of the magnet.

The scanner is driven by the magnetic interaction between permanentmagnet 206 and a proximate electromagnet 212. Electro magnet 212, in anexemplary embodiment, has a “C” shaped core 214, with a winding 216 ofsuitable impedance wound around the center section of the “C”.Electro-magnet 212 may be considered the stator of an AC motor andpermanent magnet 206 may be considered as the rotor. Core 214 may belaminated iron, as in an audio transformer or ac motor, or it may beferrite. A ferrite core is relatively lighter in weight, and providessomewhat less eddy-current losses, which in turn increases themechanical Q of the system. Electro-magnet 212 is oriented such that aline between the two pole-pieces is perpendicular to the axis of theband 202. The spacing between magnet 206 and the electro-magnetpole-pieces is not especially critical, except that the clearance shouldnot allow the magnet to contact a pole-piece under any reasonableexcursion of the magnet. Otherwise, the magnet will stick to thepole-piece, and the system will stop.

The electrical drive provided to electromagnet 212 is in the form of ashort pulse. The scanner assembly will “ring” at the mechanical resonantfrequency. Because the Q will be high (it is in the range of 100 to150), it will take several pulses for the oscillation to reach amplitudeequilibrium.

In a general oscillatory system, the drive will lag the motion by anamount approaching 90 degrees depending on the mechanical loss. Thepresent system has very little loss, so the drive pulse, at resonance,will be at the maximum velocity point i.e., 90 degrees. The presentsystem is both a motor and a generator, so any motion of the magnet willgenerate a return voltage in the electromagnet coil.

During resonance, the return signal can be shown or visualized as a sinewave 220, as shown in FIG. 9A. Sine wave 220 will in clued spikes 222 inthe center of the sine wave are the drive pulse. Sine wave 220 isproduced by attaching an oscilliscope directly to the drive coil, and aresistor was used to partially isolate the drive pulse electronics fromthe return signal. If the drive pulse rate is not at resonance, thereverse signal phase will be different from that shown in FIG. 9A. FIG.9B illustrates a return signal 224 when the system is not at resonance.It can be appreciated that spikes 225 are not centered at the peak ofthe sine wave, but are offset or skewed from the center. By comparingthe return signal just before the drive pulse to the signal just afterthe drive pulse, the phase error can be converted to a signal that canbe used to adjust the drive frequency.

A block schematic circuit 230 that will perform this automatic frequencyadjustment is shown in FIG. 8. A voltage controlled oscillator (VCO) 232provides the time-base for the system. It has a nominal frequency thatis close to the mechanical resonance. Pulses from the VCO feed a 3-bitbinary counter 234, which drives a 3-bit decode 236. The resulting 8signals in time sequence are used to control the system.

In reference to the sine wave diagram 240 shown in FIG. 8, the first twotime periods are used to collect data from the signal before the drivepulse, while the fourth and fifth periods collect data from the signaljust after the pulse. The data are collected on capacitors C1 via sampleand holds (S/H) 242. The drive pulse is generated in period three.

During periods six and seven, the signal differences, now in time sync,are transferred to capacitor C2, and to VCO 232. During period 8, theindividual capacitors C1 are discharged to help slightly with loopresponse time. The time period 3 drives a transistor 244, such as aMOSFET, that injects current into drive coil 216 via an isolatingresistor R.

The amplitude of the return signal is proportional to the peak velocity,which is proportional to the maximum scan angle, for a given frequency.Therefore, the return signal amplitude is used to provide feedback tothe drive pulse size, thereby maintaining a constant scan angle. Thenegative half-cycle of the return signal is used for this purpose. DiodeD1 and capacitor C3 provide a return signal average voltage to adifferential amplifier. A fixed set-point is supplied to the other sideof the amplifier. The amplified difference is the pulse amplitude.

The present invention also contemplates that the coil and pole structurecan be rotated about an axis defined by the pole tips. In other words,the assembly can be folded back against the frame. Such a modificationwill make the scanner assembly shorter, but a little wider in onedirection. The present invention also contemplates that two separatewindings could be placed on the electromagnet core. Two windings wouldprovide a better impedance match for the driver and separately for thereturn amplifier. It would also improve the S/N in the return signal,because the signal would be floating.

The grating is shown in FIG. 7B as a disk, but other shapes arecontemplated, such as a square or rectangular. The important point isthat the spectral resolution of a spectrometer is proportional, in part,to the width of the grating, i.e., the number of grating grooves thatare in the light beam. In production, it would be advantageous to makethe grating rotationally asymmetric, so that the grating grooves aremore certain to be mounted parallel to the rotational axis, i.e., thelong axis of the taut band.

The frame is expected to be the element that provides the primarystrength for the scanner assembly, and, therefore, it would be theelement that is fastened to the spectrometer system. The frame is shownas square. However, it could have other shapes, such as a circle, orsome combination of shapes, and could include mounting bosses orbrackets.

While the spectrometer of the present invention has been described indetail for the purpose of illustration based on what is currentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that such detail is solely for that purpose and that theinvention is not limited to the disclosed embodiments, but, on thecontrary, is intended to cover modifications and equivalent arrangementsthat are within the spirit and scope of the appended claims.

1. A spectrometer comprising: an infrared source for projecting aninfrared beam; a gas sample cell positioned in the path of the infraredbeam; a scanning mirror bearing a diffraction grating comprising aplurality of parallel lines and positioned in the path of the infraredbeam after passage thereof through the gas sample cell; a resonantscanner drive system adapted to oscillate the scanning mirror about anaxis parallel to the diffraction grating lines; a first focusing elementpositioned to focus at least one band of interest of the infrared beamas diffracted by the diffraction grating; a first detector positioned toreceive the at least one focused band of interest; and a first detectorreadout circuit operatively coupled to the first detector to receive asignal from the first detector.
 2. The spectrometer of claim 1, furthercomprising: a splitter positioned in the path of the diffracted infraredbeam from the scanning mirror; a second focusing element positioned in arespective path of the discrete bands of interest; a second detectorpositioned to receive a focused discrete band of interest; and a seconddetector readout circuit operatively coupled to the second detector toreceive a signal from the second detector.
 3. The spectrometer of claim2, wherein the splitter is a dichroic splitter for separating thediffracted infrared beam into discrete bands of interest.
 4. Thespectrometer of claim 2, wherein the first focusing element is a lens ora mirror, and the second focusing element is a lens or a mirror.
 5. Thespectrometer of claim 1, wherein the first focusing element is a lens ora mirror.
 6. The spectrometer of claim 1, further comprising a largeaperture lens disposed between the infrared source and the sample cell.